This disclosure relates to methods for pre-press color match verification and correction. Color management provides the tools to reconcile the different color capabilities of monitors, scanners, printers and printing presses to ensure consistent color throughout the production process. Color management also allows digital proofing, which is especially important now that more and more presses are run CTP (Computer To Plate) without actually generating film. Color management is based on color spaces. The range of colors, or gamut, perceived by the human eye, captured on film, displayed on a computer monitor, or rendered by a printer vary significantly. Each has its own color space, a mathematical means of describing its colors. RGB is an additive color space that combines red, green and blue light to create all other colors. RGB color spaces are used by monitors, digital camera and scanners. CMYK color, on the other hand, is a subtractive color space using cyan, magenta, yellow and black inks on paper to absorb red, green and blue light. The remaining reflected light is the color perceived by the viewer.
Both RGB and CMYK color are device dependent color spaces; i.e., the colors rendered depend on the device that produces the colors. The colors produced by a scanner vary from a monitor since a scanner uses a CCD (charge coupled device) array to capture colors, while a monitor produces colors from light-emitting phosphors. Additionally, converting an image from RGB to CMYK generally compresses the colors into a smaller gamut. To complicate matters more, the CMYK color space of one printer can vary significantly from the CMYK color space of another printer. As the graphic below shows, the colors reproducible by different mediums varies significantly.
ICC (International Color Consortium) profiles are frequently used to manage color between devices. An ICC profile is a computer file that describes the color capabilities and the color space of a particular monitor, a scanner, a printer, a printing press or a color proofing device. ICC-based color management relies on two things: device profiles, which characterize how individual devices produce color, and a color engine (also called a color matching module or CMM), which reads those profiles and translates and corrects colors between devices. ICC-based color management relies on a color space, called CIELab (or LAB), to arbitrate between the color spaces of different devices. LAB color space is based on the way the human eye perceives color and is device independent. A LAB color engine translates RGB, CMYK and other color spaces to and from LAB, which acts as an interpreter between those color spaces.
Most electronic documents to be printed or output on a particular device include multiple elements, such as text, photos, graphics and the like. Many electronic documents are a composite of other smaller documents and elements. For example, photos may be pasted into a largely text document at different locations. Color graphics and monochrome images may occur on the same page. The individual elements of an electronic document that are intended to match in color may be represented in a variety of color spaces, a situation which for example may arise because those elements are derived from prior documents of differing origins. This situation may not be immediately apparent to the user, because the colors of the objects appear to match on the display or when printed using a straightforward color transformation process, such as is typical in ICC-based color management.
One problem arises when more sophisticated color transformation is involved, such that different source color definitions take different color transformation paths. For example, if one object's color is specified in sRGB, and another object's color specified in SWOP (Specifications for Web Offset Publications) CMYK, the color processing needed to produce the device CMYK for the specific marking process may produce a different device CMYK value for the two objects. This may be due to differing black generation/black preservation strategies, even if the two objects would have matched exactly on a SWOP press (where no conversion of the SWOP CMYK would have been necessary).
Another problem arises when more sophisticated color transformation is involved, such that non-color differences in the source (e.g., differing object type) that cause the objects to take different color transformation paths. One instance of this is that the Xerox Corporation DocuSP color DFE (digital front end) can assign different ICC rendering intents to different object types. For example, by default text is assigned a “Saturation” rendering intent, while graphics are assigned a “Relative Colorimetric” rendering intent. Especially for colors near the edge or outside the printer's color gamut, the processing of the source color may produce visibly different results. Consider, for example, a print job consisting of three characters printed in a first color on a white background adjacent to three characters printed in white on a background of the first color. Using a program such as Adobe Acrobat, the first color would be displayed on a monitor for a user to view. When the job is printed using the DocuSP's color processing, the first color is slightly different.
These situations are difficult to find prior to printing without a very detailed and precise understanding of the DFE's color processing, which the typical user does not have. Consequently, color matching differences of these types have typically been discovered only upon printing, when resolution is costly and time-consuming. What is needed is a method of automatically correcting these problems. Additionally, what is needed is a method which alerts a user to color mismatch problems prior to printing and allows the user to correct any mismatches.
Proofing is a convenient means of providing a user, especially a remote user of a preview of how a particular print job will appear when printed on the selected printer. Currently, there are no color transformation utilities that mimic a DFE's color architecture remotely, i.e., independently of the production rip. Proofing color transforms are typically performed via an output ICC profile for a target printer used as a CMYK source profile on the DFE driving a proofer. Unfortunately, the target ICC profile can only characterize one print condition for CMYK image objects, and fails to correctly model any sophistication in the color processing of the target printer beyond the simple ICC workflow. What is needed is a remote proofing method for displaying a proof of a print job that reflects the remote printer's architecture.